Radiation absorbing shield for personnel and materials and method of making same

ABSTRACT

A radiation absorbing shield is constructed by forming continuous phase gelled aqueous solution, containing suitable dissolved radiation absorbing metal salts between confining walls which may be transparent. This is accomplished by mixing an aqueous solution of a polymerizable monomer, e.g., acrylamide, and an aqueous solution containing a catalyst, e.g., a redox type such as K2S2O8-Na2S2O5, and a crosslinking agent, e.g., N, Nmethylene bisacrylamide or other divinyl monomer. Alternatively a radiation source can be used in place of the free radical and crosslinking monomer solution to accomplish the polymerizing and crosslinking of the monomer.

United States Patent [191 Atkins et al.

[ RADIATION ABSORBING SHIELD FOR PERSONNEL AND MATERIALS AND METHOD OFMAKING SAME [75] Inventors: Bobby Leroy Atkins; Robert Niles Bashaw,both of Lake Jackson, Tex.

[73] Assignee: The Dow Chemical Company,

Midland, Mich.

[22] Filed: Dec. 30, 1971 21 Appl. No.: 214,394

[52] U.S. Cl 250/108 WS [51] Int. Cl. GZlf l/00 [58] Field of Search250/83 CD, 108 R,

250/108 WS; 313/61 D; 252/478 [56] References Cited UNITED STATESPATENTS 3,450,878 6/1969 Pezdirtz ct al. 250/83 CD [451 Jan. 15, 1974 IPrimary Examiner-James W. Lawrence Assistant Examiner-Davis L. WillisAtt0rney-William M. Yates et al.

[57] ABSTRACT A radiation absorbing shield is constructed by formingcontinuous phase gelled aqueous solution, containing suitable dissolvedradiation absorbing metal salts between confining walls which may betransparent. This is accomplished by mixing an aqueous solution of apolymerizable monomer, e.g., acrylamide, and an aqueous solutioncontaining a catalyst, e.g., a redox type such as K S O -Na S 'O and acrosslinking agent, e.g., N, N-methylene bisacrylamide or other divinylmonomer. Alternatively a radiation source can be used in place of thefree radical and crosslinking monomer solution to accomplish thepolymerizing and crosslinking of the monomer.

12 Claims, No Drawings RADIATION ABSORBING SHIELD FOR PERSONNEL ANDMATERIALS AND METHOD OF MAKING SAME BACKGROUND OF THE INVENTION Inworking with radioactive material it is necessary to shield personneland certain susceptible materials from the harmful effects of radiation.In the case of radioactive materials which emit neutrons it has been thepractice to use hydrocarbon polymers and compounds, e.g., polyethylene,oils, and the like, which have a high absorbency for the neutrons. Inmany situations the hydrocarbons are undesirable because of thenecessity of maintaining fire resistance in the facility since heat isevolved along with radiation emissions.

It has been the practice to use large volumes of water within confiningwalls, frequently in walls of glass or transparent plastic materials,since water is an inexpensive as well as efficient neutron absorber andhas the added ability to absorb heat and is also resistant to tire. Onedisadvantage is that the weight and consequent pressure of such largevolumes of water makes the sealing of joints at the corners and edges ofthe walls difficult. Leaking is common, particularly after exposure ofsealing materials which degrade under the influence of radiation.Another disadvantage associated with the leaking water is that, forexample, if plutonium is the material being shielded, water will reactviolently with it to cause fires if they should contact one another.

Thus, it would be highly desirable to prevent the water from leaking byimmobilizing it. A known way to immobilize water is to gel it with agelling agent. Gel

ling agents such as natural gums, synthetic polymers, e.g., polyacrylicacid, polyacrylamide, polyvinyl pyrrolidone, and the like could be used.One of the requirements is that the gelling agent be able to gel aqueoussolutions which contain appreciable concentrations, e.g., -70%, of metalsalts which augment the absorptive ability of water itself. This limitsthe selection of a gelling agent since all gelling agents do not gelsalt solutions and some are quite sensitive to even low concentrationsof ions. Crosslinked polyvinyl pyrrolidone and crosslinkedpolyacrylamide are gelling agents which can be used. These polymericgelling agents are particulate and even when in the gelled state remainparticulate swollen entities, each individual particle becomingDESCRIPTION OF THE INVENTION It has now been discovered that the aqueoussolutions can be immobilized by gelling them in situ. Thus, we havediscovered, in preparing a wall segment containing a hollow sectionfilled with water as the principal radiation absorber, the improvementwhich consists essentially of mixing 1) an aqueous solution of apolymerizable, water-soluble vinyl monomer containing an activator,e.g., an iron salt, and a crosslinker, e.g., a

water soluble divinyl monomer with (2) an aqueous solution of a freeradical producing catalyst, e.g., a redox catalyst system. Thepolymerization thus initiated produces a firm body of gel which isstable for long periods of time under the influence of radiation.

An alternate method of producing the gel is to irradiate the solution ofpolymerizable vinyl monomer in which case the radiation provides thefree-radical initiation of the polymerization and subsequently is alsocrosslinked thereby. Optionally, the divinyl crosslinking agent may beused in conjunction with a lesser amount of radiation, in which case theradiation acts only as the initiator of the polymerization.

As used herein, the term water-soluble vinyl monomer" refers to anymonomer containing vinyl unsaturation which is soluble in water at theconcentration employed and which, if it were not for crosslinking, wouldform a polymer which is soluble in water at the concentration employed.Suitable monomers include acrylamide, methacrylamide, acrylic acid,methacrylic acid, monovalent acrylate and methacrylate salts,sodiosulfoethyl (meth) acrylate, hydroxy'alkyl acrylates, copolymers ofacrylamide and acrylate salts, vinyl pyrrolidone, sodium styrenesulfonate and the like, as well as mixtures thereof.

The crosslinked polymers enumerated above are sta ble in the presence ofdissolved salts and to continuous radiation over long periods of time,e.g., several years.

The water-soluble vinyl monomer is admixed with water or, preferably,with a mixture or water and a water-soluble glycol. In general, an.aqueous solution of the water-soluble vinyl monomer is employed whichcontains from about 2 to about 50 wt. percent, preferably from about 5to about'l5 weight percent, monomer based on the total weight of thegel. A water-soluble glycol may be employed to replace a substantialportion of the water and it is generally preferred to employ from about10 to about 50 weight percent glycol based on the weight of the totalgel. As much as 65-70 percent glycol may be used. The use of a glycol toreplace a portion of the aqueous phase not only reduces the freezingpoint of the gel, but improves the clarity of the final gel. Ethyleneglycol has been found to be particularly suitable.

A water-soluble cross-linking agent is employed to provided a sufficientcross-link density in the vinyl polymer to cause said polymer to form agel with water. Suitable cross-linking agents include water-solublealkylidene bias-acrylamides such as N,N-methylenebisacrylamide, andother water-soluble divinyl compounds such as divinyl ether ofdiethylene glycol or polyvinyl compounds such as acrylate esters ofpolyols, e.g., polyethylene glycols, diglycerin and the like. Othersuitable alkylidene bis-acrylamide compounds are shown in US. Pat. No.3,046,201 to White, et a]. The term water-soluble when used with regardto the crosslinking agent means that such crosslinking agent will form aclear solution in the polymerizable aqueous mixture at the concentrationemployed. The proportion of crosslinking agent to be employed isdependent on the crosslinking density desired and on the monomerconcentration employed. A more dilute vinyl monomer solution requiresmore crosslinker to achieve the same gel properties than does a moreconcentrated monomer solution. In general, however, from about 0.01 toabout 10 weight percent, preferably from about 0.2 to about 2 percentcrosslinker is employed, based on the vinyl monomer present. If toolittle crosslinker is employed, the gel is thin and soft and will tendto flow and will not provide support for the suspended materials. If toomuch crosslinker is employed, the gel becomes brittle and losescohesiveness or forms insoluble particles which precipitate fromsolution and cloud the gel. Therefore, sufficient crosslinker isemployed to provide a gel of the desired consistency without becomingbrittle and fragile or producing insoluble particles.

Polymerization of the above-described system is produced by employing afree radical producing catalyst. Since it is desirable in most instancesthat the polymerization take place at or near room temperature, it istherefore desirable that the catalyst or catalyst system employed becapable of producing sufficient free radicals to initiate polymerizationat or near room temperature. Suitable free radical producing catalystsinclude ionizing radiation such as gamma rays, beta rays, or ultravioletirradiation, redox catalyst systems such as the mixture of an alkalimetal persulfate, and an alkali metal bisulfite activated with a solublesalt of a polyvalent metal such as FeSO Substantially any of the knownwater-soluble, free-radical producing redox catalyst systems may beemployed in the process of this invention but the catalyst composed of K5 and Na S O activated with FeSO has been found particularly effective.Still better results are achieved if the redox catalyst system isadditionally activated with a hydroperoxide such as tertiary butylhydroperoxide. This permits lower activation temperatures, more rapidpolymerization and additionally removes any color contained within thegel due to the presence of materials such as iron salts.

When radiation is used to initiate the polymerization, a dose ofionizing radiation of from about 0.001 to about 50 megarads is required.If it is desired to also crosslink by means of radiation, a total ofdoes of from about 0.01 to about 50 megarads is required. The particulardose required is dependent upon the particular monomer or monomers beingused, their concentration in solution, their being free of inhibitors,the dose rate used, and the type of radiation.

GENERAL DESCRIPTION OF THE PROCESS In a general procedure for carryingout the invention, to the water or a water-glycol solution containingthe radiation absorbing metal salt, e.g., NaBO and an iron salt(activator) is added the vinyl monomer and the divinyl crosslinkingagent. To this is added, simultaneously, aqueous solutions whichcomprise the freeradical catalyst system, e.g., solutions of X 8 0 Na SO and tert-butylhydroperoxide. These solutions are thoroughly mixed andthe polymerization proceeds rapidly. The rate of reaction is dependentupon the amount and type of catalyst, the amounts and types of divinylcrosslinker and monomer present and the temperature, which parametersare known to the artskilled. It also depends to some extent on theconcentration of the radiation absorbing salt.

The following examples are representative of the invention.

EXAMPLE 1 A solution of l 180 g. of ethylene glycol, l 180 g. acrylamideand 9,440 ml. deionized water were mixed and filtered twice. To thissolution was added 1 18 ml. of 0.1% FeSO '7H O aqueous solution, and 944ml. of 2.0% methylene bisacrylamide (MBA) aqueous solution. Thesolutions were mixed thoroughly and poured into a plastic box 24 inchesX 24 inches X 2 inches. In order to prevent the exotherm from becomingtoo great, the above quantity was divided into four equal volumes andpolymerized one after the other. Thus, each succeeding polymerizationwas carried out on top of the previously polymerized portion and eachlayer was integrally associated with the adjacent layer, forming onelarge gelled mass of aqueous solution.

For each 2,800 ml of the above solution, 118 ml. each of 1% aqueoussolutions of K S O and Na S O and 29.5 ml. ofa 1% aqueous solution oftert-butyl hydroperoxide (TBHP) were used. These three solutions wereadded simultaneously to the solution containing monomer and crosslinker.

The solutions were thoroughly mixed and polymerization proceeded rapidlywith evolution of considerable heat. The top of the box was covered witha stainless steel cover which lapped over and down the sides. A siliconerubber was used to seal the top of the box to prevent evaporation of thegelled water.

While a plastic box, or wall segment, can be used, the more usualpractice is to use a glass-walled section with metal channels joiningthe edges of the glass plates. The following example is illustrative:

EXAMPLE 2 Using l/4 inch plate glass, a window, or wall segment, wasconstructed using three panels of glass, each 24 inches X 24 inches,which were set parallel to one another in aluminum channels on three oftheir edges, leaving one side open. This provided a two compartment boxwith two side panels and a parallel central panel of glass and threesides of aluminum channel. The glass edges were sealed into the aluminumchannel using silicone rubber. Each compartment had a volume of about6.35 liters. This box was filled with gelled water in the same manner asExample 1 above. EXAMPLE 3 In order to demonstrate that thepolymerization can be accomplished in an aqueous solution containingdissolved salts, especially those capable of absorbing radiation, thefollowing experiment was conducted.

A monomer system was prepared as follows: 100 g. of dry acrylamide and100 g. ethylene glycol were dissolved in 800 ml deionized H 0 andfiltered twice. Solutions were prepared containing 2% N,N-methylenebisacrylamide (MBA), 1% K S O 1% Na S O and 1% t-butyl hydroperoxide,each dissolved in a mixture of 10% ethylene glycol and deionized water.Another solution was prepared of 0.1% FeSO in water.

A control and a sample containing 2.50 g. borax (Na B O,-10 H O)dissolved in 50 ml monomer were polymerized separately according to thefollowing procedure.

50 ml of monomer (acrylamide) 4.0 ml 2% MBA 0.5 ml 0.1% FeSO -7I'I Osoln.

2.0 ml. K 8 0 soln.

2.0 ml. Na S O soln.

0.5 ml TBHP soln.

The control began polymerizing slowly after about 2 A minutes and soonset to a clear rigid gel.

The mixture containing the borax started polymerizing after 3 or 4minutes. It proceeded slowly and as the polymerization progressed thesolution, which had been hazy, cleared. After about 30 minutes it hadcooled back to room temperature and a very tough resilient gel which hada very light amber color resulted. The clarity was good, with no bubblesor striations. The gel appeared tougher than the control.

EXAMPLE 4 A transparent solution having a grayish cast was prepared bydissolving 80 g. of ZnBr in 20 g. of water. The density of this solutionwas 2.5 g./cc. 45 g. of this solution was used to dissolve 5 g. ofcrystalline acrylamide.

This monomer solution was irradiated with gamma rays from a C source forone hour at a dose rate of 0.08 megarads per hour. The result was a firmtransparent gel.

EXAMPLE Example 4 was repeated using 5% by weight acrylamide dissolvedin a 70% ZnCl 30% water solution. The resultant transparent firm gel hada density of 2.0 g. per

EXAMPLE 6 i The following solutions were prepared:

1 (l) 22 g. of acrylamide was dissolved in 100 g. of a solution of 70%ZnCl- 30% H20.

(2) 0.2 g. of N,N-methylene bisacrylamide was dissolved in 50 g. ofsolution of 70% ZnCl 30% H 0.

(3) 0.1 g. of sodium metabisulfite was dissolved in 25 g. of 70% ZnCl30% water solution.

(4) 0.1 g. of potassium persulfate dissolved in 2 g. of water and thenmixed into 25 g. of a 70% ZnCl 30% water solution.

Solutions 1 and 2 were mixed and evacuated to remove the dissolved air.Then solution 3 was mixed in followed by solution 4. After solution 4was added, the mix set into a gel within 9 seconds. The gel was verywarm due to the exotherm during the polymerization. The resultantproduct was a transparent firm gel. EXAMPLE 7 Example 6 was repeatedusing 320 g. of 70% ZnCl 30% water solution instead of 100 g. insolution (1 This mixture set into a firm transparent gel in about 45seconds. It was felt this mixture was the easiest to handle and had thebest optical clarity.

EXAMPLE 8 y The container of gelled water prepared in Example 1 abovewas placed in a radiation chamber and exposed to'gamma radiation. Thetotal dose at the end of the experiment was 5.7 X rads and the polymerhad not changed in physical properties with the exception of a veryslight yellow discoloration. The gel was also slightly more susceptibleto being broken up with a rod than was the same non-irradiated gel.

EXAMPLE 9 A test to see the effects of freezing and thawing on a smallsample gel formation was made. This sample was prepared previously in a4 inches diameter styrene box. This was prepared from 213 ml of monomer(10% acrylamide 10% ethylene glycol in 11 0 200 ml, 8.0 ml 2% MBA soln.,2.0 ml of 0.1% FeSO '7l-l O soln. and 3.0 ml formaldehyde).

This was polymerized by adding simultaneously 8.0 ml of 1% K S O, I mlof 1% Na S O and 2.0 ml. of 1% TBHP.

About 220.5 g. of gel was tested. A clear polystyrene cover was placedover the gel (about 3/4 inch void present) and was sealed on withplastic tape. The sample was placed (cover up) in the deep freeze at Cfor 4 hours.

After 4 hours the gel was frozen solid and an opaque white in color. Itthawed very slowly at room temperature and droplets of H 0 formed on thecover (still upright). While still partially frozen approximately 5 or 6ml of water as free liquid was observed to be present on the surface ofthe gel and the plastic. When completely thawed and at room temp. therewas essentially no free liquid. The cover was removed and the containerweighed the same 252 grams. The gel was clear with slight marbeling" atthe upper surface. The optics were still good.

The cover was scaled back on and it was placed in the deep freeze for anextended test. 1t was removed from the freezer after 116 hours. Againthe gel was frozen solid and opaque. This time however, a ridge offrozen gel was noticed on the surface and ice crystals were verynoticeable in the gel. The sample was allowed to warm slowly. Again freeliquid was noted. This time the container was opened and this liquidremoved. The gel had lost 8.0 g. of water at this point. Also theresulting gel was very marbled with small tears and voids internally. Italso pulled away from the container walls at the top. The optics werepoorer, but seemed to improve with time. The gel was noticeably clearer20 hours after thawing than immediately after thawing. The sample wasobserved for several more days. No additional free liquid was noted.

EXAMPLE 10 Preparation of Monomer:

22.7 kg dry grade acrylamide was dissolved in 181.6 kg deionized H 0 and22.7 kg low conductivity grade ethylene glycol along with 370 g.N,n-methylene bisacrylamide and 4.0 g. FeSO -7H O. When dissolved andmixed this was pumped consecutively through a micron and a 10 micronfilter into a vacuum tank. A vacuum of about 740 mm Hg was pulled for 31; hours. The vacuum wasreleased and 190.7 kg of the monomer solutionwas pumped into a SS-gallon drum. A quantity of 2.27 kg of 37% aqueousformaldehyde was added to the SS-gallon drum of monomer just beforeusing.

Preparation of Catalysts:

Catalysts were prepared as follows:

64 g. Kzsgog dissolved in 6,340 ml deionized H 0 64 g. Na S O dissolvedin 6,340 ml deionized H 0 The container to be filled was a double walledstainless steel glove box. The walls were spaced 2 inches apart. The boxwas of complex design with two large holes for viewing windows. It hadfour ports for gloves, one access door and a round filter housing. Theinterior was washed several times with water and the volume determinedto be approximately 265 liters.

The demand mixing device was set up and flows set for 18.95 1./min. ofmonomer and 758 ml/min. of each catalyst (25:1 monomer to eachcatalyst). The tank was filled from the bottom of one end with theopposite end elevated. The first shot lasted approximately 11 A minuteswith flows dropping slightly. By use of small holes it was determinedthat the baffled interior was adequately filled. The remaining volume onthe top was filled the next day using freshly prepared batches ofmonomer and catalysts of the same concentrations as above. The shot wasmade from the top this time at the lower end and run over at theelevated end. When the gel was set, polyethylene and stainless steelplugs were used to seal the vents. The small door was filled at thistime also.

Water soluble salts capable of absorbing various kinds of radiation canbe employed. When water or hydrocarbons are used as shielding forneutrons, secondary radiation particles are emitted. These require additional shielding such as is provided by the addition of the saltsindicated above and enumerated and exemplified.

Salts which are useful to shield against gamma radiation are the saltsof heavy metals, e.g., Cd, W, Pb and the like, or any salts whichincrease the density of the medium. Other salts for absorbing otherradioactive emissions are known to those skilled in the art.

While not necessary to the polymerization itself, it is desirable toinclude compounds which act to inhibit fungi and bacterial growth.Formaldehyde, as shown in Example 9 above, is satisfactory for thispurpose. Other compounds useful for this purpose are the 1,3-dichloropropene salt of hexamethylene-tetramine, calcium propionate,acetaldehyde and the like. Others known to the art can be used providingthey are compatible with the system. The inhibitor compounds areunnecessary when the system is completely sterile either because of itspreparation or because of the type and intensity of radiation beingused. Sometimes the salts used for absorbing radiation are alsoinhibitors for fungi and bacteria.

We claim:

1. A radiation shield comprising a wall segment having a hollow sectionwhich is filled with water as the principal radiation absorber whereinsaid water contains a crosslinked polymer of a water soluble vinylmonomer whereby said water is immobilized.

2. The article of claim 1 wherein the water additionally contains aradiation absorbing salt.

3. The article of claim 2 wherein the salt is present at about 4 topercent based on the total weight of aqueous solution.

4. The article of claim 2 wherein the crosslinked polymer is a polymerof acrylamide and N,N- methylenebisacrylamide.

5. The article of claim 2 wherein the radiation absorbing salt is acompound of boron.

6. The article of claim 5 wherein the compound of boron is NaBO Na B O K8 0, or NH HB,O,.

7. A process for making a radiation shield which comprises providing abox-like confining vessel placing in said vessel aqueous solutions of(1) a polymerizable water soluble monomer containing an activator and acrosslinking divinyl monomer and (2) a free radical producing catalystto polymerize said monomers to form in situ a gelled aqueous solution.

8. The process of claim 7 wherein the confining vessel has at least twotransparent sides.

9. The process of claim 7 wherein the aqueous solution of polymerizablemonomer contains a radiation absorbing salt.

10. The process of claim 9 wherein the radiation absorbing salt is ZnClor ZnBr 11. The process of claim 9 wherein the radiation absorbing saltis a compound of boron.

12. A radiation shield comprising a box-like confining vessel containingan aqueous solution gelled, or immobilized, with a crosslinked polymerof a water soluble vinyl monomer and a water souble divinyl monomer, andcontaining a radiation absorbing salt dissolved therein.

mg UNITEIS STATES PATENT OFFICE e CERTIFICATE OF CORRECTION" PatentNo.,7 6,260 Dated n. 15, 19741:

inve oflg Bobby Leroy Atkins; Robert Niles Bashaw It is certified thaterror appears in the above-identified}patent and that said LettersPatent are hereby corrected as shown below Col. 2 line 30, "with mixtureor water" sheuld be f-wivith a mixture ofjwater-n Col. 2, line 49,"bias-acrylamides" should be -'-bis eerylamides n Col. 5, line 61,.xzszo jo ml" should be 5-1 6 K2920, 8.0 m1

Coi. 8, line 32, "souble" should be -soluble---.

Signed and sealed this 4th day of March 1975;]

(SEAL) I Attest: C. MARSHALL DANN Commissioner of Patents i andTrademarks JRUTH c. MASON- Attesting Officer

2. The article of claim 1 wherein the water additionally contains aradiation absorbing salt.
 3. The article of claim 2 wherein the salt ispresent at about 4 to 85 percent based on the total weight of aqueoussolution.
 4. The article of claim 2 wherein the crosslinked polymer is apolymer of acrylamide and N,N''-methylenebisacrylamide.
 5. The articleof claim 2 wherein the radiation absorbing salt is a compound of boron.6. The article of claim 5 wherein the compound of boron is NaBO2,Na2B4O7, K2B2O4 or NH4HB4O7.
 7. A process for making a radiation shieldwhich comprises providing a box-like confining vessel placing in saidvessel aqueous solutions of (1) a polymerizable water soluble monomercontaining an activator and a crosslinking divinyl monomer and (2) afree radical producing catalyst to polymerize said monomers to form insitu a gelled aqueous solution.
 8. The process of claim 7 wherein theconfining vessel has at least two transparent sides.
 9. The process ofclaim 7 wherein the aqueous solution of polymerizable monomer contains aradiation absorbing salt.
 10. The process of claim 9 wherein theradiation absorbing salt is ZnCl2 or ZnBr2.
 11. The process of claim 9wherein the radiation absorbing salt is a compound of boron.
 12. Aradiation shield comprising a box-like confining vessel containing anaqueous solution gelled, or immobilized, with a crosslinked polymer of awater soluble vinyl monomer and a water so uble divinyl monomer, andcontaining a radiation absorbing salt dissolved therein.